Doubt on Target Skew for differential pair and target lenght parameter


I have some doubts how to set the target skew. I have made some test and I have seen that when the target skew is 0, the lenght of the two nets of the differential pair are matched within 0,09mm.
However, I don’t understand how to select a different value of the target skew parameter.

Another question: is the target lenght (tune lenght command) calculated on the total lenght and not on each layer path? For example: there is a track that goes from point A to point C and the track is composed on two segments: one form point A to point B on layer TOP and the other from point B to point C on layer bottom. In this case is the target lenght calculated on the total length form point A to point C? Is it possible to lenght match each layer segment?

Thanks in advance